You can spell check in Safari when posting in online forums, thanks to its Cocoa roots. Here's how:

Highlight what you wrote and Control-click on the highlighted text.

Select Spelling from the contextual menu and then Check Spelling from the submenu.

OS X's spell checker will kick in and check spelling for you. Control-click any underlined words and select the correct spelling from the results.

[robg adds: I find it highly ironic that this hint was submitted as "Youc an spell check..." :-). In any event, this is not really a Safari feature, it's a Cocoa feature, and it's available in any Cocoa application with an input box. It would be nice, though, if (like iChat) you could permanently enable "Check Spelling as you Type." There's a menu item for it (Edit -> Spelling), but you have to select it for each input box, every time!]

And to simplify the hint even further, in Safari you can simply press Cmd+; to
have it find the next misspelling in the current text box. Pressing Cmd+;
again will move you on to the next misspelling it finds.

In iChat, you can enable spell checking all the time. To do it,
start iChat, open one chat window, enable "Check spelling as
you type," and close the window. Now quit and relaunch iChat.
Voilà! All iChat chat windows are spellchecked!